Data transmitters often implement a convolutional generator which generates a same shape as a given impulse response of a desired transmit filter. Such filters are used to reduce the high frequency content of the transmitted signals to reduce radiated emissions and to pulse shape the signal to a desired shape to reduce inter-symbol interference. When a data transmitter converter is used to perform this convolutional generation, the given impulse response is "time sliced" and switched sources which have a value of the response at a different time are implemented. Each of the switched sources is subsequently used to generate a desired output waveform. While this solution works well in most situations, the staircase approximation of the impulse response generated when a conventional data transmitter is utilized results in spurious high frequency harmonic content. For this reason, a conventional data transmitter is typically followed by a complex low pass filter which is used to smooth the output of the data transmitter. For more information on such low pass filters, refer to Modern Filter Design, by M. S. Gausi, et al., and published by Prentice Hall in 1981. The requirement that such a low pass filter be implemented increases the complexity associated with the data transmitter. Additionally, a convolutional data transmitter may be implemented to provide both a conversion and a filtering function. Implementations of convolutional data transmitters typically comprise a multi-phase analog generator which shapes an impulse response (and hence the transfer function) of the data transmitter.
The circuits and systems described above are implemented in a typical transmitter. Such traditional methods of implementing a transmitter include the steps of up-sampling a digital data stream, filtering the data stream with a digital finite impulse response (FIR) transmit filter, converting the signal to an analog signal, and smoothing the output signal with an analog low-pass filter. A traditional transmitter is well-known in the data processing art. It is also well-known that this type transmitter requires a large over-sampling factor and an accurate complex analog filter to shape a pulse provided in a pass band of the digital FIR transmit filter. This over-sampling is problematic because large oversampling rates require high speed complex digital signal processing circuits which may not be implemented in many semiconductor process due to speed, power, and area limitations. In other well-known implementations, the over-sampling factor may be eliminated, and all of the pulse shaping and filtering is performed with an analog filter as illustrated in FIG. 1. However, this second implementation requires a very complex and accurate analog filter which is difficult to implement.
Furthermore, U.S. Pat. No. 5,355,134 by Kasuga, et al., entitled "Digital to Analog Converter Circuit", describes a D/A converter, a form of digital transmitter, whose output is smoothed with the use of a first order integration operation. In the D/A converter proposed by Kasuga, et al., two D/A converters are used to provide two output currents, one of which is inverted and delayed from the other. The output currents are then summed and integrated with the use of a capacitor. While the conventional D/A converter presented by Kasuga, et al. smoothes the output of a conventional D/A converter with a first order integration, the number of D/A converters required to perform this function is more complex and requires a substantial amount of power. Furthermore, the D/A converter proposed by Kasuga, et al. only describes how to implement first order integration and is, therefore, not able to reduce a high frequency content of the signal as much as is desirable. Such a reduction in the high frequency content is only available when a second order integration is used. Additionally, a prior art reference which attempts to address this issue is U.S. Pat. No. 5,008,674 by Da Franca, et al. However, U.S. Pat. No. 5,008,674 uses switched-capacitor filters and, therefore, is only useful for lower frequencies.
Therefore, it is desirable to have a data transmitter which is able to provide a smoother converter output without the overhead associated with additional low pass filters or with multiple D/A converters and data transmitters.